The migratory eastern red bat is one of the species at risk due to wind energy projects, but researchers think there is hope for the winged creatures and renewable power to coexist. Below, the Mountaineer Wind Energy Project near Thomas, West Virginia, is site where the risk to bats first gained notice.

Photograph by Merlin D. Tuttle, Bat Conservation International

Hope for Stemming Wind Energy’s Toll on Bats

Shutting down blades on warm nights may protect migrating species

Windmills—clean, quiet, simple and endlessly renewable—may be the ultimate icons of green energy. But after sundown, their whirling blades have an unintended consequence that researchers are just beginning to understand: They kill bats by the thousands.

Their greatest impact may be on the few species of bats that migrate. Bat experts say that the problem, which peaks during migration season from July to late September, may be worse than we know—but there’s cautious optimism, too. Proposed solutions include installing speakers that blast ultrasound to drive bats away and selectively shutting off windmills when bats are most active.

Killed by the Thousands

The first hint that wind farms were killing bats came from the Mountaineer Wind Energy Center on Backbone Mountain on the ridge of the Alleghenies near Thomas, West Virginia (map). Biologists looking for birds killed by the windmills in 2003 found nearly 400 dead hoary bats and eastern red-tailed bats. They soon concluded that the West Virginia site alone was killing between 1,400 and 4,000 bats a year.

In the years since, bat experts have raised the same alarm at other wind farms in the United States and Europe. Although there have also been concerns about windmills killing birds, the problem might be more severe for bats. For reasons no one fully understands, bat species that migrate over long distances seem to be attracted to the tall windmill towers. Perhaps they mistake them for trees, or want to hunt the insects that swarm around the tall, white structures. Maybe they’re simply curious.

When they fly too close, the nocturnal creatures are often killed by windmill blades, which, at the tips, can reach speeds of more than 200 mph. These massive wing-like blades strike some of the bats, while other bats are killed when they get sucked into the low-pressure zone behind the spinning blades. The low pressure can rupture bats’ tiny lungs and hearts.

As more and more wind energy installations are built, researchers are finding proportionately more dead bats. But one problem in gauging the full scope of the carnage is that researchers have almost no idea how many bats there are. Since they fly in the dark, it’s hard to count them the way that researchers count birds. Migratory bats tend to be solitary creatures, roosting in trees and crevices, so they can’t be counted easily when they’re asleep. And they’re too small—many weigh less than an ounce—to be tracked as birds sometimes are, with tags that can pinpoint their whereabouts through the use of GPS, or global positioning system.

“No one’s ever been able to actively track these bats. Before they started showing up under wind turbines, they were very infrequently observed,” says Paul Cryan, a bat expert working for the U.S. Geological Survey in Fort Collins, Colorado. “The logistics of following animals at night without knowing where they’re going and where they’re going to land is tremendously difficult.”

That uncertainty makes it difficult to tell what kind of impact windmills are having on the overall population—and how effective efforts to reduce the number of bat kills are. “We don’t know if we’ve mitigated the effect of the kills, or if we’re just delaying a population crash for 10 or 15 years,” says Ed Arnett, director of programs at Bat Conservation International in Austin, Texas. “But if you start adding it up over time, there’s just no way the animals can sustain this.”

Indeed, the many new wind projects across the United States—enough new windmills to power 2.4 million homes were installed last year—couldn’t have come at a worse time for bats.

Biologists are already overwhelmed trying to unravel a mysterious fungus known as white-nose syndrome that’s killing hundreds of thousands of cave-dwelling bats up and down the East Coast, with no cure in sight. “Those of us charged with the well-being of bats have really had the ground fall out from under us in the last 10 years,” Cryan says.

Renewable Energy Coexistence

But new research suggests that there may be hope on the horizon for protecting the migratory bats from windmill blades, at least. New methods for reducing bat kills are being tested in the field, and initial results are promising. One option is installing speaker systems on windmills to confuse and irritate bats with ultrasound noise, a frequency too high for human hearing. “It jams them, basically,” Arnett says. “We’re flooding them with white noise, which makes it uncomfortable and disorienting airspace to be in.”

So far, experimental speaker systems have reduced the number of bat fatalities 20 to 53 percent. But there are at least two problems with ultrasound systems, which cost $20,000 per experimental unit. First, modern windmill blades cover an area the length of a football field, too far to effectively project sound at that frequency. And the long-term consequences to bats and other wildlife of constant ultrasound are unclear.

Arnett has also experimented with an obvious solution: Turn off the windmills when bats are most active. For the past two years, Arnett has been working with Iberdrola Renewables, a large Spanish-owned wind energy provider, to selectively “feather,” or shut down, wind turbine at a wind farm in Garrett, Pennsylvania (map), when wind speeds are low. “It’s a time when we are not generating a whole lot of electricity to begin with,” says Iberdrola spokeswoman Jan Johnson.

In a forthcoming study in Frontiers in Ecology and the Environment (e-View), a journal of the Ecological Society of America, Arnett reports that during peak migration season, turning off windmills on the warm, late-summer nights bats like best—nights that don’t have that much wind to harness for energy, anyway—reduced annual bat fatalities by between 44 and 93 percent. Best of all, Arnett estimated that turning off windmills on the nights when bats were most active cost only .3 percent of the wind farm’s total annual power production. “We think there are ways to refine it so there are minimal impacts on the economics,” says John Anderson, siting policy director for the American Wind Energy Association.

The idea is gaining currency. Wind energy company Invenergy earlier this year agreed to time-of-year operating restrictions as part of a plan to protect the endangered migratory Indiana bat at a project it is now building in Greenbrier County, West Virginia. A federal judge temporarily blocked that project in response to a lawsuit by environmentalists; similar action over the risk to bats has been threatened over a new wind project in Maryland.

USGS bat expert Cryan says the Iberdrola study results are a relief to those who love the embattled bats. “Curtailment offers a great hope of minimizing bat fatalities,” he says. “Finally, this is something where we can come up with a solution to the problem.”